1. Field of the Invention
The present invention relates to an apparatus for automatically adjusting a balance of brightness of a stereoscopic camera.
2. Discussion of the Background Art
In recent years, a stereoscopic vehicle surrounding monitoring apparatus using a pair of left and right cameras (stereoscopic camera having solid image element like CCD) mounted on the vehicle has been interested by automobile engineers. To detect a distance to an object, first respective picture element or pixel blocks having coincidence of brightness are found in left and right images (hereinafter referred to as stereo matching), then distance data are calculated according to the principle of triangulation from a relative deviation amount between both pixel blocks. Consequently, in order to calculate distance data with high reliability, it is necessary to balance the brightness between left and right cameras.
With respect to this, Japanese Patent Applications Laid-open No. Toku-Kai-Hei 5-114099 and No. Toku-Kai-Hei 5-265547 disclose a technique in which variations of output image signals which are caused by the difference of the output characteristic of stereoscopic cameras and the like, are corrected by referring to a lookup table. The lookup table is for changing gains and offset amounts of image signals and is stored in ROM of the system. Analogue image signals outputted from each camera are adjusted by the lookup table after being converted into digital signals by A/D converter. Thus, variations of image signals are corrected and the accuracy of the stereo matching is raised.
However, according to the aforesaid prior art, the lookup table is established individually for a given stereoscopic camera in the manufacturing process of the camera such that output characteristics of the left and right cameras agree with each other. The output characteristics of the stereoscopic camera, however, gradually deviate from the initially set value due to use environment or aged deterioration. Even if the output characteristic is well-balanced at the initial stage, that balance will be lost gradually, that is, the precision of the stereo matching degrades due to aged deterioration.